Hydraulic pumps have been commonly employed to deliver fluid under pressure to operate implement systems. It is well known to employ an additional charge pump, such as a centrifugal pump, for delivering input fluid to the hydraulic pump in order to insure "positive filling" of the pumping chambers within the hydraulic pump. One such example of a centrifugal pump used for providing "positive filling" of the hydraulic pump is set forth in U.S. Pat. No. 4,014,628 which issued on Mar. 29, 1977, to W.Z. Ruseff et al. In this arrangement, a centrifugal pump provides pressurized fluid to the inlet of the hydraulic piston pump. The centrifugal pump operates under a pressure as primarily dictated by the speed of the input drive mechanism connected to the hydraulic pump. Furthermore, as is well known, centrifugal pumps are not positive displacement pumps and may not operate at a desired controlled pressure level.
If a positive displacement charge pump is used to provide pressured fluid to the inlet of the hydraulic pump, a relief valve must be utilized to control the maximum pressure level of the fluid being delivered to the inlet of the hydraulic pump. Naturally, whenever flow is being delivered at a given pressure level, the system's power source must generate additional horsepower to drive the charge pump which provides the pressurized fluid flow to the inlet of the hydraulic pump. As main system hydraulic pumps become bigger in size, greater volume of fluid is needed to insure adequate filling of the main system hydraulic pumps. As the flow rate being generated by the charge pump increases and a constant pressure level is being utilized, additional horsepower is required to drive the charge pump. If the pressure level of the fluid flow from the charge pump is controlled, this additional horsepower could be utilized for other aspects of the system.
It is desirable to provide adequate fluid flow to the inlet of the main hydraulic pump to insure filling of the pumping chambers therein while providing only the pressure level to the fluid that is needed to properly fill the pumping chambers without consuming unnecessary horsepower when the main hydraulic pump is being operated at lower flow levels.